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Ecology and VulnerabilityAlewife

Streams where Alewife have been observed (by Mass DFW personnel) shown in orange.

Also mapped is the estimated presence and absence Alewife during the 1978-2016 Spring (green) and Fall (blue) Resource Trawling Surveys. Categories are based on the aggregation behavior of the species. Data provided by MA Division of Marine Fisheries.

Background
River herring is a term used to refer collectively to alewife and blueback herring. The two... Read More

Background
River herring is a term used to refer collectively to alewife and blueback herring. The two species are commonly referenced together as they have similar life histories and look very similar1. They are native to the Atlantic coast of North America and co-occur in rivers throughout much of their range1. The native range of alewife extends from northern Canada to South Carolina, while that of blueback herring extends from the Gulf of St. Lawrence to Florida2. Both species are anadromous, returning to their native freshwater to spawn after 3-6 years in the ocean3. Alewife use stream corridors to access ponds for spawning, although they will also use quiet reaches of rivers, whereas blueback herring prefer flowing waters for spawning3,4. Alewife spawn at cooler temperatures than blueback herring and as a result begin spawning 3-4 weeks earlier. Despite these differences, there is much overlap in spawning timing of the two species2. River herring are iteroparous, which means that they return to freshwater to spawn multiple times4. In New England, spawning may extend from late March to early June5. Adult river herring migrate to the ocean after spawning, while juveniles migrate from nursery habitats in fall of the same year6.

River herring are an important source of food for many species of birds and fish6. Historically, river herring supported thriving fisheries7. Although early dam construction, habitat degradation, and fishing resulted in large declines, a stable fishery existed through the 1960s7. Recent declines have been sudden and prompted a petition to consider the two species for listing under the Endangered Species Act (ESA), but the petition was denied and the species are unlisted8. Recent declines may be due to overharvest in marine fisheries and higher levels of natural mortality due to rebounding striped bass populations8,9.

Climate Impacts
Anadromous species, like alewife and blueback herring, use a wide range of habitats throughout their life cycle, making predictions about responses to climate change particularly complicated. Each life stage is likely to be impacted in a unique way8. However, initiation of spawning is known to be cued by temperature2. Many studies have found long-term warming trends for North American rivers over the past century10. Spring water temperatures in several New England streams have begun reaching suitable spawning temperatures about twelve days earlier than in the 1970s, implying that runs of river herring are occurring earlier10.

Warming in the marine environment has also been observed and will likely increase11. Such warming is expected to shift distributions of many fish species poleward. A similar shift has already been observed for alewife, and may continue, given predictions for future warming11.

The recent petition to list river herring under the ESA prompted a modeling study to isolate the impacts of climate change on river herring populations8. This study determined that overall, climate change is likely to impact river herring populations negatively in the Northwest Atlantic Ocean, from Cape Hatteras in North Carolina to the Gulf of Maine, with reductions in suitable habitat. However, the authors of the study were careful to note that these projections isolate the effects of ocean warming and that these effects will not occur independently of other stressors, including biotic interactions and the effects of dams.